Methods and Apparatus for Adaptive Timing for Zero Voltage Transition Power Converters

ABSTRACT

Timing circuitry causes: a first closed signal on a first switch control output before a signal on a second switch control output changes from a second closed signal to a first open signal; the first switch control output to provide a second open signal after a first selected time after the second switch control output changes from the second closed signal to the first open signal; and a third switch control output to provide a third closed signal a second selected time after the first switch control output changes from the first closed signal to a third open signal. A beginning of the first closed signal to a beginning of the first open signal is based on a later of: a current through a switch connected to the second switch control output exceeding a threshold current; and a clocked time after the beginning of the first closed signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/350,697 filed Nov. 14, 2016, which claims priority to U.S.Provisional Patent Application Ser. No. 62/322,512 filed Apr. 14, 2016,the entireties of which are incorporated herein by reference. Also, thisapplication is related to U.S. patent application Ser. No. 14/982,750(“'750 application”), the entirety of which is incorporated herein byreference.

BACKGROUND

This relates generally to electronics, and, in particular, to circuitsfor power conversion.

A category of power supplies known as switching power supplies date backseveral decades and are currently heavily utilized in the electronicsindustry. Switching power supplies are commonly found in many types ofelectronic equipment such as industrial machinery, automotiveelectronics, computers and servers, mobile consumer electronics (mobilephones, tablets, etc.), battery chargers for mobile electronics, and lowcost/light weight items such as wireless headsets and key chainflashlights. Many applications include switching power supplies forportable, battery powered devices where an initial voltage is steppeddown to a reduced voltage for supplying part of the device, such asintegrated circuits that operate at fairly low voltage direct current(DC) levels. Switching supplies are popular because these power suppliescan be lightweight and are low cost. Switching supplies are highlyefficient in the conversion of the voltage and current levels ofelectric power when compared to the prior approaches using non-switchingpower supplies, such as linear power supplies.

High efficiency is achieved in switching power supplies by using highspeed, low loss switches such as MOSFET transistors to transfer energyfrom the input power source (a battery, for example) to the electronicequipment being powered (the load) only when needed, so as to maintainthe voltage and current levels required by the load.

Switching power supplies that perform conversion from a DC input (suchas a battery) that supplies electric energy within a specific voltageand current range to a different DC voltage and current range are knownas “DC-DC” converters. Many modern DC-DC converters are able to achieveefficiencies near or above 90% by employing zero voltage transition(ZVT). The ZVT technique was developed by Hua, et. al. and is describedin a paper published in 1994 (“Novel Zero-Voltage-Transition PWMConverters,” G. Hua, C.-S. Leu, Y. Jiang, and F. C. Lee, IEEE Trans.Power Electron., Vol. 9, No. 2, pp. 213-219, March 1994), which isincorporated by reference in its entirety herein. The use of the ZVTfunction in DC-DC converters reduces energy loss that would otherwiseoccur due to switching losses. ZVT also has the additional benefit ofreducing voltage stress on primary power switches of the DC-DCconverters. Reduction in voltage stress on a switch allows the switch tohave a lower voltage tolerance rating and, therefore, potentially theswitch can be smaller and less costly.

The ZVT circuitry employed by prior DC-DC converters introducesadditional switches and corresponding additional energy loss and voltagestress on switching elements. However, the impact of energy loss andvoltage stress of the ZVT function is much less significant than theoverall performance improvements to the switching converters that employZVT functionality. Further improvements to reduce energy loss andvoltage stress of the ZVT function are still needed. These improvementswill permit improvement of electronic equipment in increased batterylife, lower cost of operation, and improved thermal management.

SUMMARY

Timing circuitry is configured to cause: a first closed signal on afirst switch control output before a signal on a second switch controloutput changes from a second closed signal to a first open signal; thefirst switch control output to provide a second open signal after afirst selected time after the second switch control output changes fromthe second closed signal to the first open signal; and a third switchcontrol output to provide a third closed signal a second selected timeafter the first switch control output changes from the first closedsignal to a third open signal. A beginning of the first closed signal toa beginning of the first open signal is based on a later of: a currentthrough a switch connected to the second switch control output exceedinga threshold current; and a clocked time after the beginning of the firstclosed signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a ZVT DC-DC buck powerconverter.

FIG. 2 is a timing diagram for a sequence of switch transition events tooperate ZVT functionality.

FIG. 3 is a timing diagram of the sequence of switch transition eventsto operate ZVT functionality for an example arrangement of the presentapplication.

FIG. 4 is a group of waveform plots with the timing diagrams of FIG. 3.

FIG. 5 is a circuit diagram of an ideal equivalent circuit diagram ofthe ZVT resonant circuit.

FIG. 6 is a circuit diagram of an ideal equivalent circuit diagram ofthe ZVT resonant circuit in an alternative arrangement.

FIG. 7 is a circuit diagram of a ZVT buck converter circuit includingcontrol elements.

FIG. 8 is a series of graphs showing the effect on the switch nodevoltage under different levels of adjustment.

FIG. 9 is a graph showing the effect of input voltage on the ZVTprocess.

FIG. 10 is a circuit diagram of the loop detection unit.

FIG. 11 is a flow chart showing the operation of the two loops of thezero voltage transition (ZVT) functionality of the circuit of FIG. 7.

FIG. 12 is a circuit diagram including a controller that provides a ZVTpower converter in a buck circuit topology that incorporates thearrangements of the present application.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the drawings, corresponding numerals and symbols generally refer tocorresponding parts unless otherwise indicated. The drawings are notnecessarily drawn to scale.

In this description, the term “coupled” may include connections madewith intervening elements, and additional elements and variousconnections may exist between any elements that are “coupled.”

To better illustrate the shortcomings of the prior ZVT approaches,circuit 100 of FIG. 1 illustrates a ZVT DC-DC converter arranged in abuck converter circuit topology. Buck DC-DC converters provide an outputvoltage at a lower voltage than an input voltage. Other types of DC-DCconverters that can benefit from the use of ZVT switching include, butare not limited to, boost converters that increase voltage to the loadto a voltage greater than the input voltage, and buck-boost DC-DCconverters that dynamically transition between the buck and boostfunctions to adapt to various input voltage levels (having inputvoltages that could be either greater or less than the output voltage)to provide an output voltage to the load.

FIG. 1 illustrates in a simplified circuit diagram the switchingelements, key passive components, and key parasitic elements of a ZVTDC-DC buck converter circuit 100. Omitted from FIG. 1 for simplicity ofexplanation are minor components, minor parasitic elements, the circuitsfor monitoring output voltage, and the control circuit for controllingthe switch timing that are utilized in example ZVT DC-DC buck powerconverters.

In FIG. 1 circuit 100 includes two primary power switches, 102 (S1) and104 (S2), that in conjunction with the output inductor 106 (Lo) andcapacitor 108 (Co) perform the primary function of the buck converter.The buck converter circuit 100 supplies energy to the load (representedas a resistor 110 (Ro)) at an output voltage level Vo that is a reducedvoltage from the DC input voltage supply 112 (Vin). Vin represents boththe external element that is the source of input voltage (such as abattery or another power supply) to the ZVT power converter and thevoltage level across the positive and negative terminals of the Vininput voltage source.

Auxiliary switches Sa1 and Sa2 and auxiliary inductor La are thecomponents that are added to the conventional switching convertertopology to accomplish the ZVT functionality. A primary parasiticinductance that contributes to voltage stress on switch S2 isrepresented in FIG. 1 by parasitic inductance 114 (Lbyp). The sourceterminal of transistor 102, the drain terminal of transistor 104 and oneterminal of each auxiliary inductor 116 (La) and the output inductor 106(Lo) are coupled as illustrated in FIG. 1 to a common switch node 118(Switch Node). The first auxiliary switch 120 (Sa1), the secondauxiliary switch 122 (Sa2), and the auxiliary inductor 116 are coupledtogether at auxiliary node 124 (Aux Node). All four switches in examplecircuit 100 of FIG. 1 (S1, S2, Sa1, and Sa2) are shown implemented asenhancement mode n-channel MOSFETs. Drain-to-source parasiticcapacitances of switches S1 and S2 are important to the circuitdescription and are illustrated in FIG. 1 as capacitance 126 (Cds1) andcapacitance 128 (Cds2), respectively. The intrinsic body diode of MOSFETswitches is also shown connected between source and drain for allswitches (S1, S2, Sa1, and Sa2) of FIG. 1.

While enhancement mode n-channel MOSFETs are commonly used as switchesin DC-DC converters as shown in the example in FIG. 1, other types oftransistor switches as well as diode switches have been employed and canbe used to form the circuit 100. The switches in FIG. 1 can also be usedto form other types of switching power converters.

Circuit 100 supplies a reduced voltage to the load (the output voltageis across resistor 110 (Ro)) by alternatively switching between twoprimary states. In one of the primary states (defined by switch S1closed and switch S2 open, which means switch S1 is a transistor that isturned on, while switch S2 is a transistor that is turned off), theinput voltage source (Vin) supplies energy to the load, and energy tomaintain or increase magnetic energy is also stored in inductor Lo. Inthe other primary state (defined by switch S1 open and switch S2 closed,which means that switch S1 is a transistor that is turned off, whileswitch S2 is a transistor that is turned on), current flow from theinput voltage (Vin) is blocked. In this state, the magnetic energypreviously stored in inductor Lo is converted to electric energy, andsupplies energy to the load (resistor Ro). The output voltage across theload Ro is maintained in a pre-defined range by varying the relativeamount of time the circuit spends in each of the primary states.

Converters that alternate between the two states described hereinaboveare sometimes described as pulse width modulated (PWM) switchingconverters. This description is used because the output voltage Vo isproportional to the input voltage Vin, multiplied by the duty cycle ofswitch S1 (a ratio of the on time of switch S1 to the total cycleperiod). Typically, prior known buck converters cycle between thesestates (often at frequencies such as hundreds of kHz to 1 MHz andabove). In addition to the two primary states, there are brief deadtimes during the transitions between the two primary states. During thedead times, switches S1 and S2 are simultaneously open, that is thetransistors implementing switches S1 and S2 are simultaneously turnedoff. Dead times are used to insure there is not a high current pathacross the input voltage source (Vin) directly to ground, which couldoccur if both switches S1 and S2 are simultaneously closed. ConventionalPWM switching power supplies employ two dead times during each cycle ofoperation: a first dead time occurs when switch S1 opens and ends whenswitch S2 closes; and a second dead time occurs when switch S2 opens andends when switch S1 closes.

In a ZVT converter, such as circuit 100, the ZVT function begins beforethe beginning of the second dead time with S2 opening, and the ZVTfunction ends after the second dead time ends with switch S1 closing.The ZVT function does not operate in the first dead time of the buckconverter cycle described above (the time between switch S1 opening andS2 closing).

FIG. 2 illustrates in a timing diagram the sequence of switch transitionevents used to operate ZVT functionality in the buck converter circuit100. In FIG. 2, the switching events are labeled t0, t1, t3, and t4.(Note that there is no event labeled t2 in FIG. 2, for increasingsimplicity of explanation when comparing the switching event sequence ofthe conventional ZVT DC-DC buck converters with the switching eventsequences of example arrangements of the present application.) In FIG.2, the dead time described hereinabove during the time interval betweenswitch S2 opening and switch S1 closing begins at event t1 and ends atevent t3.

The open and closed states of each of the four switches (primary S1, S2,and auxiliary switches Sa1, and Sa2) illustrated in FIG. 1 arerepresented in FIG. 2 by the voltage applied to the switch gates (Vg1,Vg2, Vga1, and Vga2 respectively) and shown in four graphs: 232; 234;236; and 238. Graph 232 illustrates the voltage on the gate of switchS1, graph 234 illustrates the voltage on the gate of switch S2, graph236 illustrates the voltage on the gate of switch Sa1, and graph 238illustrates the voltage on the gate of switch Sa2. A voltage annotatedas Von applied to a switch gate indicates the switch is closed (thecorresponding transistor is on), and a voltage annotated as Voffindicates the switch is open (the corresponding transistor is off). FIG.2 illustrates a sequence of switching events, and does not illustratespecific voltage levels, waveform shapes, and time increments.

ZVT functionality for prior known approaches begins at event labeled t0in FIG. 2 with switch Sa1 turning on, as shown in graph 236. In the timeleading up to event t0 switch S2 has been closed, and switches S1 andSa2 have been open for a significant portion of the current buckconverter cycle. Time progresses from event t0 to event t1 illustratedin FIG. 2. At time t1, switch S2 opens as shown in graph 234. At thenext event, t3, switches S1 and Sa2 close as shown in both graphs 232,238. Switch Sa1 opens at time t3, as shown in graph 236, and after ashort delay to provide a dead time, Sa2 closes just after event t3, asshown in graph 238. At event t4, Sa2 opens as shown in graph 238 tocomplete ZVT functionality for the current cycle of the buck converter.

The example conventional ZVT buck converter circuit 100 illustrated inFIG. 1 accomplishes ZVT when the primary power switch S1 transitionsfrom open to closed (S1 turn on as shown in graph 232) at event labeledt3 illustrated in FIG. 2. Switch S1 turns on at t3 with zero or nearzero volts across it. For the circuit 100 to reach a condition with zeroor near zero volts across switch S1 before S1 turning on (or closing),an L-C resonant circuit is used. The L-C resonant circuit increases thevoltage at the source terminal of switch S1 (coupled to the node “SwitchNode” in FIG. 1) until the voltage is approximately equivalent to thevoltage at the drain terminal of S1, which is coupled to andapproximately equivalent to the input voltage, Vin. The L-C resonantcircuit includes the auxiliary inductor La and the parallel combinationof capacitances Cds1 and Cds2 (the drain to source parasiticcapacitances of the switches S1 and S2 respectively) (see FIG. 1). ThisL-C resonant circuit is referenced herein as the “ZVT resonant circuit.”The ZVT resonant circuit is a portion of circuit 100. In someapproaches, the ZVT resonant circuit resonates only when switch Sa1 isclosed and switches S1, S2, and Sa2 are open, which is during the timespan between events t1 and t3 in FIG. 2. The time span between events t1and t3 for some approaches is equivalent to one-quarter cycle of theresonant frequency of the ZVT resonant circuit.

While some conventional DC-DC converters incorporating the ZVT functiontypically have lower energy loss and lower voltage stress on transistorswitches when compared to DC-DC converters formed without the ZVTfunction, the ZVT function itself introduces additional energy loss andvoltage stress.

There are two key contributors to energy loss of prior known ZVTfunctions that are reduced by use of the arrangements of the presentapplication. First, energy is lost when auxiliary switch Sa1 turns offwhen conducting peak current as it transitions through the MOSFET linearregion. The second key contribution to energy loss during the ZVToperation is the sum of conduction losses through the auxiliary switchesSa1, Sa2, the primary switch S1, and inductor La.

The most significant impact of voltage stress resulting from the ZVTfunction is on the voltage tolerance required for switch S2. Voltagestress on switch S2 impacts S2 transistor size and potential cost. Thevoltage stress on switch S2 is the result of switch Sa1 turning off withpeak current flowing through it, causing a voltage spike across switchS2 induced by the parasitic inductance 114 (Lbyp). In addition, there isa voltage spike across Sa1 when it turns off with current flowingthrough it, due to ringing with parasitic inductances. However, sizingSa1 for higher voltage tolerance is not a significant impact topotential converter cost, since Sa1 is already a relatively smalltransistor when compared to the primary power transistors, S1 and S2.

As discussed above, FIG. 1 illustrates in a simplified circuit diagramthe switching elements, key passive components, and key parasiticelements of a ZVT DC-DC buck power converter. For the purposes ofsimplification, minor components, minor parasitic elements, and thecircuits for monitoring output voltage and controlling the switch timingthat are present in prior approaches and example arrangements of thepresent application are omitted from FIG. 1. An aspect of thearrangements of the present application is the sequencing and timing oftransitions for the switches depicted in circuit 100. Consequently,circuit 100 is used herein for explanation of the switching events of aZVT DC-DC buck power converter as well as for the illustration ofarrangements of the present application.

In arrangements of the present application, the switch transitionsequencing and timing employed results in improved power efficiency. Useof the arrangements also enables improved ZVT power converters withreduced semiconductor die area for switch implementation.

The switch transition sequencing and timing employed in the arrangementsof the present application occurs during the operation of the ZVTfunction, and does not significantly impact the operation of circuit 100during the remainder of the power supply cycle. Consequently, adescription of the full power supply cycle is not included.

FIG. 3 illustrates in a timing diagram the sequence of switch transitionevents to operate ZVT functionality for an example arrangement of the'750 application. In FIG. 3, the switching events are labeled t0, t1,t2, t3, and t4.

The open and closed states of each of the four switches (S1, S2, Sa1,and Sa2) illustrated in FIG. 1 are represented in FIG. 3 by the voltageapplied to the switch gates (Vg1, Vg2, Vga1, and Vga2 respectively).Graph 332 illustrates the voltage Vg1 at the gate terminal of switch S1.Graph 334 illustrates the voltage Vg2 at the gate terminal of switch S2.Graph 336 illustrates the voltage at the gate terminal of the switchSa1. Graph 338 illustrates the voltage at the gate terminal of switchSa2. A voltage annotated as Von applied to a switch gate indicates thatthe switch is closed because a transistor is on, and a voltage annotatedas Voff indicates the switch is open because a transistor is off. Graphs332, 334, 336 and 338 in FIG. 3 illustrate the sequence of switchingevents. FIG. 3 does not illustrate specific voltage levels, waveformshapes, and time increments. For both the arrangements of the presentapplication and for other ZVT approaches there is a brief dead timebetween switch Sa1 turn off and switch Sa2 turn on. This dead time isused to insure there is not a high current path across the input voltagesource, Vin. The dead time between switch Sa1 turn off and switch Sa2turn on does not significantly impact circuit 100 functionality.Consequently, switch Sa1 turn off, the intervening dead time, and switchSa2 turn on are illustrated as occurring in a single event (at time t2)in FIG. 3 for further simplicity of explanation.

ZVT functionality for the example arrangements of the '750 applicationbegins with the event labeled t0 in FIG. 3, with switch Sa1 turning on,as shown in graph 336, while switch S2 remains closed (on) and switchesS1 and Sa2 remain open. In FIG. 3, time progresses to event t1. At eventt1, switch S2 opens as shown in graph 334. At the next event, t2, asshown in FIG. 3, switch Sa1 opens as illustrated in graph 336, and aftera short delay that fulfills the dead time requirement, switch Sa2 closesas shown in graph 338. (In sharp contrast to the arrangements of thepresent application, in prior approaches, the ZVT circuits do not employa switching event at time t2, as previously stated.) As shown in FIG. 3,at event t3 for the arrangements of the present application, switch S1is closing as is illustrated in graph 332. At event t4, switch Sa2 opensas shown in graph 338 to complete ZVT functionality for the currentcycle of the buck converter.

Additionally, the waveform and timing diagrams provided herein are notannotated with voltage and current values and time increments sincespecific values depend on a how a specific example arrangement isimplemented. When waveforms are compared herein, the same relativevoltage, current, and time scales are used.

For each successive span of time between the above stated switchingevents, a description of the ZVT functionality and the switch transitionsequencing and timing employed by the arrangements of the presentapplication within the respective time span follows, as well as acomparison of the present arrangement to prior approaches. In addition,a description of the circuit functionality to control the switchsequencing and timing of the arrangements of the present application isprovided hereinbelow.

The first time span during the operation of the ZVT function is betweenevents t0 and t1 as shown in FIG. 3. The ZVT function starts during eachbuck converter cycle at event t0. In the time leading up to t0, the ZVTfunction begins in a state with switch S1 open and switch S2 closed, andswitches Sa1 and Sa2 are open. At event t0, switch Sa1 closes, allowingcurrent to flow through the auxiliary inductor La, which ramps from zeroamperes until the current flowing in inductor La is approximatelyequivalent to the current flowing through inductor Lo. Simultaneously,the current flowing in the closed switch S2 ramps to zero or near zero.The behavior of circuit 100 for both the arrangements of the presentapplication and for the other ZVT approaches is similar for the timeinterval starting at event t0 and ending at event t1, except that thetime at which event t1 occurs after event t0 is adjusted by the controlcircuit of the arrangements of the present application. The adjustmentsare further described hereinbelow.

The adjustment to the time at which event t1 occurs can be performed inorder to modify the resonant trajectory of the ZVT resonant circuit,such that the switch node voltage will be equal or nearly equal to theinput voltage, Vin, at event t3 (ZVT functionality for subsequent eventsis described below). Adjusting the resonant trajectory on an on-goingbasis allows the ZVT function to adapt to dynamic changes in the loadand for other operating conditions. The adjustment to the time at whicht1 (following the events at t0) occurs is accomplished in thearrangements indirectly by monitoring and adjusting the current Is2flowing through switch S2 when it is turned off at event t1. Toaccomplish the adjustment of the S2 turn off current, the switch nodevoltage is measured at event t3. If the switch node voltage is equal toor greater than Vin at time t3, the target value (the current through S2when S2 turned off, or IS2-off) for the S2 turn off current isincrementally reduced. If the switch node voltage is less than Vin attime t3, Is2-off is incrementally increased. During the operation of theZVT function of the immediately following buck converter cycle, thecurrent in switch S2 is monitored between events t0 and t1 and iscompared to Is2-off (set in the previous cycle). In the arrangements,the switch S2 is turned off when the current Is2 is equal to or lessthan Is2-off.

The second time span during the operation of the ZVT function as shownin FIG. 3 is between events t1 and t2. For both the arrangements of thepresent application and for other ZVT approaches, switch S2 opens atevent t1 with zero or near zero current flowing through it, as shown ingraph 334. Switches S1 and Sa2 remain open at t1. With only switch Sa1closed, the inductor La resonates with the parallel combination of theparasitic drain to source capacitances, Cds1 and Cds2, of switches S1and S2, respectively (the ZVT resonant circuit). In example arrangementsof the present application, event t2 occurs at a time that is ⅙ tr afterevent t1 (where “tr” is the resonant period of the ZVT resonantcircuit). At ⅙ tr, the switch node reaches a voltage greater than ½ Vin.At time t2, Sa1 is opened and Sa2 is closed (after a short dead timedelay between opening Sa1 and closing Sa2) as shown in FIG. 3 in graphs336, 338.

FIG. 4 illustrates in graphs 440, 442 and 444 the current in auxiliaryinductor 116 (La, FIG. 1), labeled I(La), for the example arrangementsof the '750 application and also presents graphs comparing the currentobtained to the corresponding current obtained in other approaches forconventional ZVT converters. The switching events t0, t1, t2, t3, and t4shown in FIG. 4 are duplicated from FIG. 3 in graphs 432, 434, 436 and438, respectively, for clarity of illustration. The time scales of FIG.4 for I(La) waveforms are the same for both the arrangements of thepresent application and the prior approaches illustrated for comparison.

Graphs 432, 434, 436, and 438 of FIG. 4 correspond to the graphs 332,334, 336 and 338 in FIG. 3, respectively, and depict the gate voltageson the switches S1, S2, Sa1, and Sa2, respectively, for circuit 100 inFIG. 1. In FIG. 4 an example sequencing arrangement of the '750application is illustrated at the events t0, t1, t2, t3 and t4.

In FIG. 4, the current flowing in the inductor La (labeled 116 inFIG. 1) is shown on separate graphs 440 for I(La) with the event time t2adjustment and 442 for I(La) without t2 adjustment, as well as graph 444which combines both the arrangements on the same set of axes. Graph 444is presented to illustrate that arrangements with t2 adjustment operateat lower inductor La current for a shorter time period during the timespan between events t2 and t4. For the overlaid waveform diagram ingraph 444, a dashed line is used to illustrate current I(La) without t2adjustment to show where the waveforms differ significantly. In graphs440, 442 and 444 of FIG. 4, the current through Lo is represented byfixed grid line labeled I(Lo). In practice, I(Lo) is not a fixed valueand is load dependent. For simplicity of explanation, I(Lo) is shown asa fixed value.

An additional difference between approaches that do or do not adjust t2is that in the arrangements where t2 is adjusted, a voltage spike occurswhen switch Sa1 opens at event t2 with current flowing through it, dueto ringing with parasitic inductances. In other ZVT buck converterswhere t2 and t3 coincide, this voltage spike appears only across switchS2, since it is open and switch S1 is closed when the spike occurs. Incontrast, in the arrangements where t2 is adjusted, the arrangementsoperate by opening switch Sa1 with both S1 and S2 open and before thedrain to source capacitance of S1 (Cds1) is fully discharged,distributing the voltage spike across both switches S1 and S2 in series.Specifically, in the approach where t2 is adjusted, the seriescombination of the parasitic drain-source capacitances Cds1 and Cds1 ofswitches S1 and S2 respectively form a capacitive divider across whichthe voltage spike occurs. Dividing the voltage spike across both S1 andS2 reduces the voltage tolerance requirement of switch S2 (when comparedto the voltage tolerance requirement for the same switch in otherapproaches). The voltage tolerance requirement of the switch S1 is notincreased with t2 adjustment, because the spike across S1 that occurswhen Sa1 opens in the example arrangements is less than the voltageacross S1 at other times during the operation of the buck converter.

The third time span during the operation of the ZVT function for theapproach with t2 adjustment is between events t2 and t3. As statedhereinabove in the description of FIG. 3, event t2 for the arrangementsof the '750 application occurs when the transition of switch Sa1 fromclosed to open occurs, and switch Sa2 transitions from open to closedshortly afterwards, with switches S1 and S2 remaining open. When switchSa1 opens and switch Sa2 closes, the ZVT resonant circuit configurationis changed and the voltage across inductor La reverses. Current flowthrough inductor La will continue in the same direction, and resonancewill continue on a different trajectory with the current in Laresonating towards zero, resulting in the switch node continuing tocharge. The energy stored in La at event t2 continues charging theswitch node until it becomes approximately equivalent to the inputvoltage Vin, provided the event at time t2 occurs with the switch nodevoltage still sufficiently above ½ the Vin voltage level. It should benoted that for an ideal circuit, if t2 were to occur when the switchnode is exactly ½ Vin, then the energy stored in inductor La will chargethe switch node voltage to Vin. However, in the example arrangements, t2should occur with the switch node at a voltage greater than ½ Vin so asto accommodate component parameter variance and non-ideal circuitcharacteristics. The switch node voltage becomes approximatelyequivalent to Vin at a time that is 1/12 tr after the event t2, at whichtime event t3 occurs, with S1 closing. This sequence is shown in graphs432, 434, 436, and 438 at time t3.

FIG. 5 illustrates in a simplified circuit diagram an equivalent idealZVT resonant circuit 500 for the example configuration operating duringthe span of time from event t1 to t2 described hereinabove. FIG. 6illustrates in another simplified circuit diagram the equivalent idealZVT resonant circuit 600 for the example configuration for the span oftime from event t2 to t3 described hereinabove. Both equivalent circuits500 and 600 illustrate a portion of circuit 100 of FIG. 1 with switchesS1, S2, Sa1, and Sa2 in the states described hereinabove for therespective time spans. For simplicity, in the diagrams for circuits 500and 600, the switches Sa1 and Sa2 are treated as ideal and shown asinterconnect conductors when closed, and are simply not shown when open.

As described hereinabove, during the time period between events t2 andt3 for arrangements of the present application, stored energy ininductor La is used to charge the switch node from a level greater than½ Vin to Vin. In sharp contrast to the present arrangements, for ZVTconverters using other approaches, the converters utilize energy fromthe power converter input voltage source, Vin, to charge the switch nodeto be approximately equivalent to the input voltage, Vin. Consequently,more energy is stored in La and current is higher in La when switch S1closes at t3 during operation of prior approaches (than for thearrangements of the present application). Greater stored energy in Laand higher current through La result in greater energy losses for theother approaches.

As stated hereinabove, the event t2 of the present arrangements is notpart of the operation of other approach converters. Therefore, otherapproach ZVT resonant circuits continue resonance on the same trajectoryfor the full time span from t1 to t3. In contrast, for the examplearrangements herein described, the resonant trajectory is modified atevent t2 as described hereinabove.

As illustrated in FIG. 4, compared to other approaches, current throughswitch Sa1 is lower when Sa1 turns off during operation of examplearrangements of the '750 application. The current through Sa1 is lowerdue to ramping the switch node voltage to a level greater than ½ Vin.The turn-off of switch Sa1 is performed early (when compared to theother approaches), as opposed to waiting for the switch node voltage tobe approximately equivalent to Vin. As a result, energy lost by switchSa1 while it is conducting in the transistor linear region (during thetransition from on to off) is much lower for arrangements of the presentapplication.

The fourth and final time span during the operation of the ZVT functionis between events t3 and t4. During the period of time between events t3and t4, switch S1 turns on at event t3, and the current in inductor Laramps down to zero, at which time Sa2 is turned off at event t4, endingthe operation of the ZVT function for the current buck converter cycle.After switch S1 closes, the portion of the current in stored in inductorLa that exceeds the current in Lo is returned to the source and theremainder of the current in La flows into Lo to supply the load.

There are at least three differences between the operations of otherapproaches and the operation of the arrangements of the '750 applicationin the time period between events t3 and t4. The first difference isthat switch Sa1 opens and switch Sa2 closes at t3 in other approaches.For the approaches of the '750 application, Sa1 opens and Sa2 closesbefore the event t3 (at t2) as described hereinabove. The seconddifference is that a smaller fraction of the energy stored in inductorLa is returned to the source (when compared to the other approaches),thus reducing energy losses. The third difference is that for the otherapproaches, the inductor La current reaches its peak at t3. Instead, forthe approach of the '750 application, the peak current through La islower and the peak current is achieved earlier in time (at event t2),resulting in the time period from t3 to t4 being significantly shorterfor the described arrangements. Additionally, the time from t2 to t4 forthe described arrangements is shorter than the time from t3 to t4 forother approaches.

The operation of example arrangements of the '750 application describedhereinabove results in switches Sa1, Sa2, and S1 and inductor La eachconducting current for shorter amounts of time (when compared to theother approaches) with lower RMS current levels, resulting insignificantly lower energy loss. The benefits that can accrue by use ofthe arrangements include: RMS current through Sa1, Sa2, S1, and La arelowered, since Sa1 turns off before the switch node voltage reachingVin, resulting in lower peak current in La, Sa1, and Sa2; conductiontime for switch Sa1 is reduced, since it turns off earlier than in priorapproaches, turning off before the switch node voltage reaching Vin;and, since the peak current in La is lower for the arrangementsdescribed hereinabove, the current in La ramps to zero in less time,resulting in lower RMS current in switch S1. In addition, since thecurrent in La ramps to zero more rapidly, the conduction times forswitch Sa2, switch S1, and inductor La are also reduced.

FIG. 7 is a diagram of a ZVT buck converter circuit 700 includingcontrol elements for controlling the operation of the switches in theZVT buck converter to form an arrangement of the present application.Similarly labeled elements of FIG. 7 perform similar functions to thoseof FIG. 1. That is, elements 702, 704, 706, 708, 710, 712, 716, 718,720, 722, 724, 726, and 728 perform similar functions to elements 102,104, 106, 108, 110, 112, 116, 118, 120, 122, 124, 126, and 128,respectively, in FIG. 1. The timing of the operation of circuit 700during the interval from when S2 turns off until S1 turns on is shown inFIG. 3. Elements 750 through 768 control the gates of switches S1 (702),Sa1 (720) and S2 (704) as further described hereinbelow.

Elements 750 through 768 include components that implement two feedbackloops that control the timing of switches S1, S2 and Sa1. The firstfeedback loop includes switch node monitor 750, adaptive threshold unit752, Vin feedforward unit 756, Is2-off reference 758, current monitor760 and comparator 762. This feedback loop determines when to shut offswitch S2 (event t1 in FIG. 3) based on the current Is2 through switchS2. The second feedback loop includes switch node monitor 750 andadaptive overlap delay unit 754. This loop determines when to shut offswitch S2 based on an adaptive time delay. The second feedback loop isused when the load 710 (Ro) is drawing so little current that the firstloop cannot be used to accurately set the timing of circuit 700. Loopdetection unit 764 determines which of these two feedback loops sets thecontrol timing as further explained hereinbelow.

With regard to the first feedback loop, switch node monitor 750 capturesthe voltage at the switch node when S1 turns on at the end of the S2-onto S1-on gap (from t1 to t3 in FIG. 3). The goal is to make the switchnode voltage V_(sw) at this time as close to Vin as possible. Adaptivethreshold unit 752 compares Vin to the switch node voltage V_(sw). IfV_(sw) is less than Vin, the base Is2-off reference is incrementedhigher. If V_(sw) is greater than Vin, the base Is2-off reference isdecremented lower. The new base Is2-off reference is then used for thenext cycle of converter circuit 700 at the end of the S2-on to S1-ongap.

Vin feedforward unit 756 compensates for fluctuations of the inputvoltage Vin. Using the feedforward of the voltage Vin avoids thesituation where a temporary fluctuation of Vin causes a large adjustmentto the Is2-off reference. Such fluctuations can occur, for example, whena starter motor of a car pulls a large amount of current from thebattery or with other temporary side loads to supply 712. When thefluctuation is over, circuit 700 must then adjust back to near theoriginal value of Is2-off reference. The need to adjust back to theoriginal value will cause circuit 700 to have many cycles where theIs2-off reference is not correct for proper operation of circuit 700.During this time, the switch node voltage will be significantly higherthan Vin or lower than Vin. During this time, circuit 700 will operateinefficiently, requiring more robust specifications for switch S1.

FIG. 8 is a series of graphs 846-849 showing the effect on the switchnode voltage V_(sw) under different levels of adjustment. Only theevents at times t1, t2 and t3 (FIG. 3) are shown for clarity. In thesegraphs, it is assumed that Vin is 10V and thus the goal for V_(sw) at t3is 10V. Graph 846 shows when S2 turns off and when S1 turns on. Graph847 shows an ideal case for voltage V_(sw) where V_(sw) reaches 10V att3. In graph 848, V_(sw) reaches 10V too soon, thus wasting energy asV_(sw) overshoots the target voltage of 10V. The graph 848 only shows amild over voltage at t3 because the voltage is clamped by the body diodeof S1. However, when the switch node voltage reaches Vin before t2, thiscauses excess voltage stress on S2. In graph 849, switch S2 is turnedoff too soon. Thus, the ZVT circuitry does not have time to reach thedesired level of V_(sw). This adds to power loss because of the currentsurge through S1 that is due to the difference of V_(in) and V_(sw).

As noted hereinabove, the Vin feedforward unit 756 (FIG. 7) compensatesfor fluctuations of Vin. FIG. 9 is a graph 900 showing the effect of Vinon the ZVT process. When switch Sa1 (720 in FIG. 7) turns on at t0 (FIG.3), the current through inductor La (716 FIG. 7) begins rising on aslope that is proportional to Vin. In FIG. 9, t_(prop) represents apropagation delay including the combined delays of the currentcomparator and S2 driver turn-off. The beginning of t_(prop) is when thecurrent comparator needs to trip. The end of t_(prop) is when thecurrent in S2 is equal to IS2-off. Vin3>Vin2>Vin1 in FIG. 9. Therefore,Vin3/La has a greater slope than Vin2/La, and Vin2/La has a greaterslope than Vin1/La. At this point the resonant effect discussed abovewill continue the rise of the current through inductance La to reach thegoal current of Is2 shown in FIG. 9, which will place the voltage at theswitch node (718 in FIG. 7) at the desired voltage.

The shut off of switch S2 is determined by a comparison of the currentthrough S2 and the Is2-thres reference. Sa1 is shut off ⅙ tr after S2 isshut off. The proper current Is2-thres for each value of Vin is shown inFIG. 9, where the slope of each line crosses the beginning of t_(prop)(i.e. at t2). These current values are labeled Is2-thres1, Is2-thres2and Is2-thres3, which correspond to the voltages Vin1, Vin2 and Vin3,respectively. As shown by FIG. 9, the proper value of Is2-thres changeswith the level of Vin. The correction can be determined mathematicallyusing the formula in Equation 1:

$\begin{matrix}{{i_{s\; 2\; {off}}} = {{{i_{{s\; 2} -}{thres}}} - {\frac{V_{in}}{La}t_{prop}}}} & \lbrack 1\rbrack\end{matrix}$

Where t_(prop) is a propagation delay including the combined delays ofthe current comparator and S2 driver turn-off. This adjustment isperformed by Vin feedforward unit 756 and provided to Is2-off reference758.

The second loop in the example arrangement of FIG. 7 starts with theswitch node monitor 750, which monitors the voltage at switch node 718at the time switch 702 (S1) turns on. Adaptive overlap delay unit 754includes an overlap time from t0 to t1. That is, the time when S2 andSa1 are both turned on (i.e. the transistor on times overlap). Asfurther explained hereinbelow, the second feedback control loop onlyapplies when load current through load 710 (Ro) is small, such that thefirst feedback control loop cannot accurately determine the time betweent0 and t1. Switch node monitor 750 compares the voltage at switch node718 (V_(sw)) at the point when S1 turns on to conduct the voltage Vinand adaptive overlap delay unit 754 adjusts the overlap time based onthe output of switch node monitor 750. If V_(sw) is less than Vin, theoverlap time is increased. If V_(sw) is more than Vin, the overlap timeis decreased. Adaptive overlay delay unit then compares the overlap timeto a clocked time after t0. A comparator compares the time after t0 tothe overlap delay, as is further explained with regard to FIG. 10hereinbelow.

As noted hereinabove, the second feedback control loop of FIG. 7 onlycontrols the circuit under very light loads, which means that currentIs2 is very small. Loop detection unit 764 determines which loopcontrols. FIG. 10 is a circuit diagram of an example implementation thatcan be used for the loop detection unit 764. Event t1 is triggered bythe ZVT_BEGIN signal, which is the output of AND gate 1072. One of theinputs of AND gate 1072 is the output of comparator 762, which providesa high or “one” output when Is2 is greater (less negative) thanIs2-thres. The other input to AND gate 1072 is from comparator 1070,which is part of overlap delay unit 754. Comparator 1070 provides a highor “one” output when the time after t0 (t-t0) is greater thant_(ovlp-thres). Therefore, t1 is triggered when both comparators 762 and1070 provide a high or “one” output. Under most loads, current Is2 willcross Is2-thres well after the time after t0 passes t_(ovlp-thres).Thus, the time when the ZVT operation begins (when ZVT_BEGIN is high ora “one”) is essentially controlled by comparator 762. However, undervery light loads, Is2 current is always below (that is, less negative orsmaller absolute value) than Is2-thres. When the load current is verylight, very little current through inductor 716 La (FIG. 7) drives theswitch node 718 (FIG. 7) to Vin. Under these very light loads, currentIs2 is always higher (less negative) than Is2-thres. In this case,comparator 762 provides a “one” output before comparator 1070.Therefore, the ZVT_BEGIN signal is under the control of comparator 1070.In addition, the inputs to AND gate 1074 are the output of comparator762 and the output of comparator 1070 as inverted by inverter 1076. Theoutput of AND gate 1074 is the OVLP_TRIP signal. Therefore, this signalis only a “one” when the output of comparator 762 is high while theoutput of comparator 1070 is low, i.e., when overlap delay unit 754 andthe second loop are in control. OVLP_TRIP will only be a “one” while theoutput of comparator 1070 is low. Accordingly, the OVLP_TRIP signalshould be latched in most applications (so that it can be provided toother control functions).

Returning to FIG. 7, loop detection unit 764 provides a signal to turnoff switch 704 (S2) according to the determined controlling loop (atevent t1 in FIG. 3). This signal also begins the timer for delay 766. Inan example arrangement, delay 766 provides a delay of ⅙ tr; that is, ⅙of the cycle time of at the resonant frequency of the resonant circuitshown in FIG. 5. After the delay period of delay 766, switch 720 (Sa1)is turned off (event t2 in FIG. 3) and the timer for delay 768 begins.In an example arrangement, delay unit 768 provides a delay of 1/12 tr;that is, 1/12 of the cycle time of at the resonant frequency of theresonant circuit shown in FIG. 6. Not shown is a short delay from theoutput of delay 766 controlling the turn on of switch 722. As explainedhereinabove, this dead time delay is to prevent a direct short that mayoccur if switch 720 (Sa1) and switch 722 (Sa2) were on at the same time.After this time period, switch 702 (S1) is turned on (at event t3) andthe buck converter cycle begins again.

FIG. 11 is a flow chart 1100 showing the operation of the two loops ofthe zero voltage transition (ZVT) functionality of circuit 700 (see FIG.7). At step 1102 the ZVT process begins with turning on switch Sa1 (720in FIG. 7). As noted in step 1104, the turn on of Sa1 is at t0 (in FIG.3). In step 1106 loop detection unit 764 determines which loop willcontrol, as explained hereinabove. As explained above, under very lightload conditions, the measured current Is2 is immediately greater (lessnegative) than Is2-thres. In this case, t_(ovlp-thres) determines whenswitch S2 (704 in FIG. 7) is turned off, as shown in step 1108. SwitchSa1 (720 in FIG. 7) is then turned off and switch S1 (702 in FIG. 7) isturned on after the respective time delays as shown in step 1110. Instep 1112, the switch node voltage V_(sw) is compared to Vin. If V_(sw)is greater than Vin, t_(ovlp-thres) is decremented for the next cycle,as shown in step 1114. If V_(sw) is less than Vin, t_(ovlp-thres) isincremented for the next cycle, as shown in step 1116. In step 1118,switch S1 is turned off and switch S2 is turned on, in accordance withthe duty cycle of circuit 700 (FIG. 7). The method in the flow diagramin FIG. 11 then returns to step 1102.

As explained hereinabove, when load conditions are not light, themeasured current Is2 will become greater than Is2_thres aftert_(ovlp-thres). In this case, in step 1106, the comparison of thecurrent Is2 to Is2-thres made by comparator 762 (in FIG. 7) determineswhen switch S2 (704 in FIG. 7) is turned off, as shown in step 1120.Switch Sa1 (720 in FIG. 7) is then turned off and switch S1 (702 in FIG.7) is turned on after the respective time delays as shown in step 1122.In step 1124, the switch node voltage V_(sw) is compared to Vin. IfV_(sw) is greater than Vin, Is2_thres is decremented for the next cycle,as shown in step 1126. If V_(sw) is less than Vin, Is2_thres isincremented for the next cycle, as shown in step 1128. Then, in step1118, switch S1 is turned off and switch S2 is turned on, in accordancewith the duty cycle of circuit 700 (FIG. 7). The method in the flowdiagram then returns to step 1102.

FIG. 11 only illustrates aspects of switch sequencing and timing controlfor the ZVT part of the power converter cycle, and does not illustratethe sequencing and timing control for the entire ZVT function or for theremaining operations of the power converter.

FIG. 12 depicts in another block diagram of a circuit 1200 including acontroller 1280 that provides a ZVT power converter in a buck circuittopology incorporating arrangements of the present application. In anaspect, controller 1280 can be formed as a monolithic integrated circuitor a multichip package, which may or may not include other componentsshown in FIG. 12. Similarly labeled elements of FIG. 12 perform similarfunctions to those of FIG. 7. That is, elements 1202, 1204, 1206, 1208,1210, 1212, 1216, 1218, 1220, 1222, 1224, 1226, and 1228 perform similarfunctions to elements 702, 704, 706, 708, 710, 712, 716, 718, 720, 722,724, 726, and 728, respectively, in FIG. 7. In circuit 1200, the examplebuck converter of FIG. 1 is again shown, with an input voltage Vin, apair of primary switches S1, S2, which with the output inductor Lo,capacitor Co, and resistance Ro, provide a voltage Vout to a load Rocoupled to the output. To provide the zero voltage transition functionfor the converter, auxiliary switches Sa1 and Sa2, and inductor La, areused to control the voltage at the source terminal of switch S1 and toallow switch S1 to be turned on when the source-drain voltage isapproximately zero.

In FIG. 12, a controller 1280 provides the gate control voltages Vg1,Vg2 to the primary switches S1, S2 and also the gate control voltagesVga1, Vga2, to the auxiliary switches Sa1, Sa2. Controller 1280implements the switching sequences to operate the buck converter ofcircuit 1200 including the delayed turn off of the auxiliary switch Sa1,and the delayed turn on of switch S1 after that event, switchingsequences that are used in the arrangements of the present applicationto improve the performance of the ZVT converter. Controller 1280 alsocontrols the gate voltages for other portions of the converter operatingcycle to regulate the output voltage. The inputs to controller 1280include the input voltage, Vin, the output voltage, Vout, the switchnode voltage, V_(sw), and the current Is2 (or a voltage equivalent)provided by current monitor 1260. Among other functions, controller 1280performs the functions of elements 750, 752, 754, 756, 758, 762, 764,766 and 768 of FIG. 7 described hereinabove.

Controller 1280 can be implemented in a variety of ways, for example ascircuits including, as non-limiting examples, a microcontroller,microprocessor, CPU, DSP, or other programmable logic, as a dedicatedlogic function such as a state machine, and can include fixed or userprogrammable instructions. Further, as an alternative arrangement,controller 1280 can be implemented on a separate integrated circuit,with the switches S1, S2, Sa1, Sa2, and the remaining passive analogcomponents, implemented on a stand-alone integrated circuit. In analternative, one or more of switches S1, S2, Sa1, Sa2, and the remainingpassive analog components may be implemented in the same substrate ascontroller 1280. Controller 1280 can be implemented as an applicationspecific integrated circuit (ASIC), using field programmable gate arrays(FPGAs) or complex programmable logic devices (CPLDs) and the like. Thesequencing and timing control of the novel arrangements can beimplemented as software, firmware or hardcoded instructions. Delay linesand counters and the like can be used to determine the delays ⅙ tr, 1/12tr, as determined by a particular hardware designer. Because thearrangements herein are implemented as changes in the sequence of gatesignals applied to the transistors of a converter, the arrangements canbe utilized in existing converter circuits by the modification ofsoftware and some sensing hardware, and thus the arrangements can beused to improve the performance of prior existing systems without theneed for entire replacements of the converter hardware.

In an example aspect, an integrated circuit includes a third switchcontrol output; a second switch control output; a first switch controloutput; a fourth switch control output; and a switch node voltage input.Timing circuitry causes a first closed signal on the first switchcontrol output before a signal on the second switch control outputchanges from a second closed signal to a first open signal. The timingcircuitry causes the first switch control output to provide a secondopen signal after a first selected time after second switch controloutput changes from the second closed signal to the first open signal.The timing circuitry causes the third switch control output to provide athird closed signal a second selected time after the first switchcontrol signal changes from the first closed signal to a third opensignal. The timing circuitry determine timing from a beginning of thefirst closed signal on the first switch control output to the beginningof the first open signal on the second switch control output based on alater of an overlap time and a current through a switch connected to thesecond switch control output exceeding a threshold current.

In another example aspect, the integrated circuit adjusts the overlaptime based on a comparison of the of a supply voltage level at onecurrent handling terminal of a first switch connected to the thirdswitch control output and a measured voltage at a second currenthandling terminal of the switch before the third closed signal.

In another example aspect, the integrated circuit adjusts the thresholdcurrent based on a comparison of the of a supply voltage level at onecurrent handling terminal of a first switch connected to the thirdswitch control output and a measured voltage at a second currenthandling terminal of the switch before the third closed signal.

In yet another example aspect, the timing circuit causes a fourth closedsignal on the fourth switch control output a third selected time afterthe second open signal.

In another example aspect, the second and third selected times are basedon a resonant cycle time of a resonant circuit including an auxiliaryinductance, an inherent capacitance of a first switch connected to thethird switch control output and an inherent capacitance of a secondswitch connected to the second switch control output port.

In yet another example aspect, the integrated circuit controls a buckconverter.

In another example aspect, at least one switch controlled by one of thefirst, second, third and fourth switch control output ports is formed ina same substrate as the integrated circuit.

In another example aspect, a switch coupled to at least one of the thirdswitch control output port, second switch control output port, firstswitch control output port, and fourth switch control output port is afield effect transistor.

In another example aspect, an integrated circuit includes a third switchcontrol output; a second switch control output; a first switch controloutput; a fourth switch control output; and a switch node voltage input.Timing circuitry causes a first closed signal on the first switchcontrol output before a signal on the second switch control outputchanges from a second closed signal to a first open signal. The timingcircuitry causes the first switch control output to provide a secondopen signal after a first selected time after second switch controloutput changes from the second closed signal to the first open signal.The timing circuitry causes third switch control output to provide athird closed signal a second selected time after the first switchcontrol signal changes from the first closed signal to a third opensignal. The timing circuitry determines timing from a beginning of thefirst closed signal on the first switch control output to the beginningof the first open signal on the second switch control output based on acurrent through a switch connected to the second switch control outputexceeding a threshold current, in which the threshold current isadjusted as a function of a voltage level of a voltage supply.

In another example aspect, the voltage supply has one terminal coupledto a first current handling terminal of a switch controlled by the thirdswitch control output and the voltage supply has a second terminalcoupled to a second current handling terminal of a switch controlled bythe second switch control output port.

In yet another example aspect, the timing circuitry determines thetiming from a beginning of the first closed signal on the first switchcontrol output to the beginning of the first open signal on the secondswitch control output based on a later of an overlap time and thecurrent through the switch connected to the second switch control outputexceeding the threshold current.

In another example aspect, the timing circuit causes a fourth closedsignal on the fourth switch control output a third selected time afterthe second open signal.

In yet another example aspect, the first, second and third selectedtimes are based on a resonant cycle time of a resonant circuit includingan auxiliary inductance and an inherent capacitance of a first switchconnected to the third switch control output and an inherent capacitanceof a second switch connected to the second switch control output port.

In another example aspect, the integrated circuit controls a buckconverter.

In another example aspect, at least one switch controlled by one of thefirst, second, third and fourth switch control output ports is formed ina same substrate as the integrated circuit.

In yet another example aspect, a switch coupled to at least one of thethird switch control output port, second switch control output port,first switch control output port, and fourth switch control output is afield effect transistor.

In another example aspect, a method of controlling a power converterincludes executing a plurality of cycles. Each cycle includes turning ona first switch during a first period, the first switch having a firstcurrent handling terminal coupled to a first terminal of a power supplyand a second current handling terminal coupled to a terminal of a firstinductor, the first inductor having another terminal coupled to a firstterminal of an output load. Each cycle also includes turning on a secondswitch during a second period, the second period occurring after thefirst period such that the first switch and second switch are not onsimultaneously, the second switch having a first current handlingterminal coupled to the second current handling terminal of the firstswitch and a second current handling terminal coupled to a secondterminal of the power supply and a second terminal of the output load.Each cycle also includes turning on a third switch at a first timeduring the second period and turning the third switch off at a secondtime after the second period but before a beginning of the first periodof a succeeding cycle, a first current handling terminal of the thirdswitch coupled to the first terminal of the power supply and a secondcurrent handling terminal coupled to a first terminal of a secondinductor, a second terminal of the second inductor coupled to the secondcurrent handling terminal of the first switch. Each cycle also includesturning on a fourth switch on at a third time after the second time andturning the fourth switch on during the first period of the succeedingcycle, the fourth switch having a first current handling terminalcoupled to the first terminal of the second inductor and a secondcurrent handling terminal connected to the second terminal of the powersupply. The second period ends at a third time period after the firsttime based on a later of an overlap time and a current through a switchconnected to the second switch current handling terminal exceeding athreshold current.

In another example aspect, the threshold current is adjusted in responseto changes in a voltage provided by the power supply.

In another example aspect, the overlap time is adjusted based on acomparison of a voltage at a beginning of the first period on the secondcurrent handling terminal of the first switch is to a voltage providedby the power supply.

In yet another example aspect, the threshold current is adjusted basedon a comparison of a voltage at a beginning of the first period on thesecond current handling terminal of the first switch is to a voltageprovided by the power supply.

What is claimed is:
 1. An integrated circuit comprising: a first switchcontrol output; a second switch control output; a third switch controloutput; and timing circuitry configured to cause: a first closed signalon the first switch control output before a signal on the second switchcontrol output changes from a second closed signal to a first opensignal; the first switch control output to provide a second open signalafter a first selected time after the second switch control outputchanges from the second closed signal to the first open signal; and thethird switch control output to provide a third closed signal a secondselected time after the first switch control output changes from thefirst closed signal to a third open signal; in which timing from abeginning of the first closed signal to a beginning of the first opensignal is based on a later of: a current through a switch connected tothe second switch control output exceeding a threshold current; and aclocked time after the beginning of the first closed signal; and inwhich the clocked time is adjusted based on: a supply voltage level at afirst current handling terminal of a first switch connected to the thirdswitch control output; and a measured voltage level at a second currenthandling terminal of the first switch before the third closed signal. 2.The integrated circuit of claim 1, wherein the threshold current isadjusted based on the supply voltage level and the measured voltagelevel.
 3. The integrated circuit of claim 1, further comprising a fourthswitch control output, wherein the timing circuit is configured to causea fourth closed signal on the fourth switch control output a thirdselected time after the second open signal.
 4. The integrated circuit ofclaim 3, wherein the first, second and third selected times are based ona resonant cycle time of a resonant circuit including an auxiliaryinductance, an inherent capacitance of the first switch connected to thethird switch control output and an inherent capacitance of a secondswitch connected to the second switch control output.
 5. The integratedcircuit of claim 1, wherein the integrated circuit controls a buckconverter.
 6. The integrated circuit of claim 1, further comprising afourth switch control output, wherein a switch coupled to at least oneof the first, second, third and fourth switch control outputs is formedin a same substrate as the integrated circuit.
 7. The integrated circuitof claim 1, further comprising a fourth switch control output, wherein aswitch coupled to at least one of the first, second, third and fourthswitch control outputs is a field effect transistor.
 8. An integratedcircuit comprising: a first switch control output; a second switchcontrol output; a third switch control output; and timing circuitryconfigured to cause: a first closed signal on the first switch controloutput before a signal on the second switch control output changes froma second closed signal to a first open signal; the first switch controloutput to provide a second open signal after a first selected time afterthe second switch control output changes from the second closed signalto the first open signal; and the third switch control output to providea third closed signal a second selected time after the first switchcontrol output changes from the first closed signal to a third opensignal; in which timing from a beginning of the first closed signal to abeginning of the first open signal is based on a later of: a currentthrough a switch connected to the second switch control output exceedinga threshold current, in which the threshold current is adjusted based ona supply voltage level at a first current handling terminal of a firstswitch connected to the third switch control output; and a clocked timeafter the beginning of the first closed signal, in which the clockedtime is adjusted based on the supply voltage level and a measuredvoltage level at a second current handling terminal of the first switchbefore the third closed signal.
 9. The integrated circuit of claim 8,further comprising a voltage supply to provide the supply voltage level,the voltage supply having: a first terminal coupled to the first currenthandling terminal of the first switch; and a second terminal coupled toa current handling terminal of a second switch controlled by the secondswitch control output.
 10. The integrated circuit of claim 8, furthercomprising a fourth switch control output, wherein the timing circuit isconfigured to cause a fourth closed signal on the fourth switch controloutput a third selected time after the second open signal.
 11. Theintegrated circuit of claim 10, wherein the first, second and thirdselected times are based on a resonant cycle time of a resonant circuitincluding an auxiliary inductance and an inherent capacitance of thefirst switch connected to the third switch control output and aninherent capacitance of a second switch connected to the second switchcontrol output.
 12. The integrated circuit of claim 8, wherein theintegrated circuit controls a buck converter.
 13. The integrated circuitof claim 8, further comprising a fourth switch control output, wherein aswitch coupled to at least one of the first, second, third and fourthswitch control outputs is formed in a same substrate as the integratedcircuit.
 14. The integrated circuit of claim 8, further comprising afourth switch control output, wherein a switch coupled to at least oneof the first, second, third and fourth switch control outputs is a fieldeffect transistor.